Histological Study of Suprabranchial Chamber Membranes in Anabantoidei and Clariidae Fishes

Accessory respiratory organs (AROs) are a group of anatomical structures found in fish, which support the gills and skin in the process of oxygen uptake. AROs are found in many fish taxa and differ significantly, but in the suborder Anabantoidei, which has a labyrinth organ (LO), and the family Clariidae, which has a dendritic organ (DO), these structures are found in the suprabranchial cavity (SBC). In this study, the SBC walls, AROs, and gills were studied in anabantoid (Betta splendens, Ctenopoma acutirostre, Helostoma temminckii) and clariid (Clarias angolensis, Clarias batrachus) fishes. The histological structure of the investigated organs was partially similar, especially in relation to their connective tissue core; however, there were noticeable differences in the epithelial layer. There were no significant species-specific differences in the structure of the AROs within the two taxa, but the SBC walls had diversified structures, depending on the observed location. The observed differences between species suggest that the remarkable physiological and morphological plasticity of the five investigated species can be associated with structural variety within their AROs. Furthermore, based on the observed histology of the SBC walls, it is reasonable to conclude that this structure participates in the process of gas exchange, not only in clariid fish but also in anabantoids.

Structure of the gill during reproduction in the unionids Anodonta grandis, Ligumia subrostrata, and Carunculina parva texasensis

This study characterizes the morphological modifications of the gill that establish the functional isolation of the central water channels into brood chambers during reproduction. The anodontids use the entire lateral demibranch for reproduction. The water channels are permanently subdivided by additional septa, resulting in more channels, but they are about one-third the size of those in the medial gill. Immediately before egg deposition, each water channel is subdivided by the formation of two small secondary water channels that receive water from the water canals, and a large brood chamber with no direct exposure to the circulating pond water. The secondary water channels disappear 1 or 2 weeks after release of the larvae. The septal tissue at the suprabranchial chamber becomes enlarged after egg deposition and seals the dorsal opening of the brood chamber from the exhalant water. The lampsilids do not form secondary water channels and the eggs are deposited in the central water channel of the posterior half of the lateral demibranch. The septal tissue enlarges at the suprabranchial chamber after egg deposition. In addition, muscular tissue near the ostial opening of the canal and at the canal – channel junction is able to constrict the canal orifice. In all species studied, the modified gill allows little if any ambient pond water into the brood chamber, and the aqueous environment is under the control of the maternal tissues.

Biomodal Gas Exchange During Variation in Environmental Oxygen and Carbon Dioxide in the Air Breathing Fish Trichogaster Trichopterus

Gas exchange in the gourami, Trichogaster trichopterus, an obligate air breather, is achieved both by branchial exchange with water and aerial exchange via labyrinth organs lying within the suprabranchial chamber. Ventilation of the suprabranchial chamber, MOO2, MCOCO2, gas exchange ratios of both gills and labyrinth organs, and air convection requirements have been measured under conditions of hypoxia, hyperoxia or hypercapnia in either water or air. In undisturbed fish in control conditions (27 °C), air breathing frequency was 12 breaths/h, gas tidal volume 30 μl/g, total oxygen uptake 5.2 μ.M/g/h and total carbon dioxide excretion 4.1 μM/g/h, indicating a total gas exchange ratio of approximately 0.8. The aerial labyrinth organs accounted for 40% of oxygen uptake but only 15% of carbon dioxide elimination. Hypoxia, in either inspired water or air, stimulated air breathing. Total MOO2 was continuously maintained at or above control levels by an augmentation of oxygen uptake by the labyrinth during aquatic hypoxia or by the gills during aerial hypoxia. Hypoxia had no effect on MCOl partitioning between air and water. Hypercapnia in water greatly stimulated air breathing. About 60% of total MCOCO2 then occurred via aerial excretion, a situation unusual among air breathing fish, enabling the overall gas exchange to remain at control levels. Aerial hypercapnia had no effect on air breathing or O2 partitioning, but resulted in a net aerial CO2 uptake and a decrease in overall gas exchange ratio. Trichogaster is thus an air breathing fish which is able to maintain a respiratory homeostasis under varying environmental conditions by exploiting whichever respiratory medium at a particular time is the most effective for O2 uptake and CO2 elimination.

The hypobranchial gland in the Bivalvia

The hypobranchial gland of the protobranchs Nucula nucleus and Solemya parkinsoni comprises mucous cells flanked by inversely conical and ciliated regenerative cells. A similar structure occurs in the filibranch Monia squama and in the eulamellibranchs Fimbria fimbriata and Corbicula fluminea except that in the two latter the conical cells are not ciliated and in C. fluminea they are also secretory.The primitive function of the hypobranchial gland was the consolidation, in a mucous stream, of waste material for expulsion via the exhalant aperture. The evolution of the filibranch and eulamellibranch ctenidia dividing the mantle cavity into infrabranchial and suprabranchial chambers with waste material removed via the inhalant aperture abrogated the need for such a structure. It has, however, been retained in some filibranchs where the ctenidia are possibly less efficient.In Nucula delphinodonta the hypobranchial gland secretes a brood pouch attached to the shell, Enhancement of this function in lamellibranch bivalves, notably C. fluminea and possibly F. fimbriata, has resulted in the gland functioning as an organ for the nutrition of developing larvae incubated in the suprabranchial chamber. An epithelium with regenerative cells of different structure occurs in the similarly incubatory Sphaerium corneum.